The present invention relates to a method and arrangement for measuring and indicating the field strength of a signal received by a mobile station operating in a cellular network. The invention also relates to a mobile station in a cellular telephone system, comprising means for sending and receiving signals, means for measuring the field strength of a signal, and means for reading data into a storage means in the mobile station. The invention also relates to a cellular radio system that comprises switching centers, base station controllers, base stations and mobile stations, and in which the base stations are provided with means for generating messages and for sending said messages to mobile stations, and the mobile stations are equipped so as to operate on a certain service level and receive said messages.
In cellular telephone systems the conditions of the locations of the different mobile stations in a cell are different with respect to the location of the base station. Some mobile stations are located near the base station without interfering obstructions between them. On the other hand, a mobile station may be located on the outskirts of a cell, or there may be an obstacle between it and the base station that attenuates the signal, or signals of other mobile stations may interfere with the operation of the mobile station. Mobile stations located in poor conditions have to use high transmission power so that their signals be on an acceptable level as they arrive in the base station. If the conditions are good, on the other hand, a high transmission power unnecessarily drains the battery of the mobile station and, at the same time, the mobile station""s signal unnecessarily interferes with the signals of the other mobile stations in the same cell. The transmission power applied by the mobile station can be controlled by the base station if the service provider so wishes, see e.g. Michelle Mouly and Marie-Bernadette Pautet: The GSM System for Mobile Communications, chapter 6.1.5.1. In the GSM system, for example, the transmission power of a mobile station, like that of a base station, can be changed by about 30 dB in 2-dB steps at intervals of 60 ms. The control information is based on data obtained from a received signal level measurement carried out by the base station and mobile station.
The mobile station measures and indicates the received field strength. The measured field strength can be displayed to the user as a graphic indication on the mobile station. On the basis of the measurement result the mobile station decides whether or not the measured field strength meets the requirements for the connection. The prior-art field strength measurement indication was developed during the early stages of GSM (Global System for Mobile communications) standards when all that were used were mainly the basic services, such as speech transmission and slow data transmission. At the moment, however, there are additional services available that can be utilized in a GSM cellular network. These include e.g. fast data transmission at 14.4 kbps, general packet radio service (GPRS), and high speed circuit switched data (HSCSD) services. For example, the error correcting coding used on the faster 14.4 kbps channel had to be reduced so that a greater amount of data can be transferred on the channel. Along with increased data transmission rates it has been necessary to tighten up the field strength requirement on the connection so that errors occurring in the signal on the transmission path can be reduced to such an extent that the channel coding in use is able to correct the errors. The mobile station, however, measures and indicates the field strength as an absolute value just like before. Points P1 and P2 in FIG. 1 show, by way of example, the field strength indicated by the mobile station in accordance with FIGS. 2a and 2b. In FIGS. 2a and 2b the field strength is shown on the display unit of the mobile station 20 using e.g. a vertical piecewise-continuous line 22, 23 above the symbol depicting an antenna 21. If, for example, the vertical line on the display unit 20 of the mobile station shows five indicator blocks 22, like in FIG. 2b, the field strength is optimal for GSM use. If the display shows only one indicator block 23, like in FIG. 2a, the field strength value is near the lower limit of operation, but the link is still acceptable. If there are no field strength indicating blocks at all above the antenna symbol 21 or the antenna symbol is missing, the field strength is insufficient for communication and no link can be established.
When using absolute field strength indication, it is possible to have a situation in which the field strength indication 23 indicates sufficient field strength but in reality the communication link will not function as desired with the service selected by the user. In the case of FIG. 1, two different operating ranges can be drawn, by way of example, around the base transceiver station (BTS) such that the outer circle 11 meets the GSM basic operation requirements at 9.6 kbps. However, the other GSM service shown in FIG. 1, fast data transmission at 14.4 kbps, can only be accomplished inside the inner circle 12, say, at point P2, since the channel coding used on a fast transmission link cannot correct all errors generated on the transmission path when the mobile station is located at point P1. In the situation depicted by FIG. 1, it is difficult for the user, while at point P1, to understand why the fast data transmission will not work although the prior-art field strength indicator in the mobile station indicates that the field strength is sufficient for transmission, as shown in FIG. 2a. At point P2 the prior-art absolute field strength indication is in accordance with FIG. 2b, and at P2 the both services will work because the channel coding used on the faster transmission channel is able to correct the errors possibly generated on the transmission channel.
To remove the problem, various improvements have been proposed in order to enhance the services provided by the GSM system. U.S. Pat. No. 5,732,328 discloses a method in which the mobile station is controlled so as to change its transmission power in accordance with the information class of the signal itself, such as speech, data, or video, for instance. According to experience, the error tolerances of these different information classes are different. For example, normal speech transmission tolerates interference and even momentary disconnections better than pure data transmission. Statistically, then, the transmission power of the mobile station can be controlled according to the information contents of its signal. Data transmission, for example, needs higher transmission power than speech transmission.
The transmission power of a mobile station may also be controlled according to a method disclosed in U.S. Pat. No. 5,710,981 without direct control from the base station. The signal received by the mobile station described in said patent publication is degraded by means of a circuit in the mobile station in a controlled manner until it reaches a level where the quality of the signal is unacceptable. Since the mobile station has received information about the transmission power of the base station, it can now make a calculation of the transmission path losses and, based on that, the transmission power of the mobile station itself can be set as desired with respect to power consumption, signal quality and interference caused.
However, the basic problem remains. The mobile station measures and, if necessary, indicates the absolute value of the measured field strength. The field strength can be adjusted in the methods described above by changing the transmission power of the mobile station when necessary. However, the need for additional power is different for the different value-added services offered in the cellular network in question and, therefore, one single absolute measurement value will not provide sufficient data to form a basis for the decision about whether the field strength in question is sufficient for establishing the desired service link which may comprise several different value-added services. Often a mobile station employing the prior-art methods makes an attempt to establish the link and only after an unsuccessful attempt it is found that the field strength is insufficient.
An object of the invention is to reduce said disadvantages associated with the prior art.
The method according to the invention is characterized in that the value representing the field strength of the received signal is calculated at least partially on the basis of the service used by the mobile station.
The mobile station according to the invention is characterized in that it comprises means for calculating and indicating a value representing the field strength of the received signal as well as means for setting a minimum value for the field strength of the signal received by the mobile station on the basis of the service used by the mobile station.
The base station according to the invention is characterized in that it comprises means for communicating a service-specific value (Off) needed by mobile stations to calculate a value representing the field strength of the received signal to the mobile stations and means for controlling the transmission power on the basis of the service-specific values (Off).
The cellular network according to the invention is characterized in that a cellular radio system comprises accessible information about the field strength service-specific values (Off) allowed in the cellular network and said system is arranged so as to transmit said information from a base station to a mobile station for the calculation and indication of a value representing the field strength of the received signal.
Some preferred embodiments of the invention are disclosed in the dependent claims.
The main idea of the invention is as follows: Instead of measuring and indicating the absolute field strength the mobile station uses a virtual field strength dependent on the data contents of the signal. A mobile station registered to a service fetches from its memory an offset value to adjust the absolute field strength value. This way it is found out whether the desired service can be used with the current measured absolute field strength value. The transmission power used by the mobile station is controlled in a service-based manner so that it meets the requirements set on the field strength. Thus the mobile station will know, and can also indicate to the user, if necessary, prior to the establishment of the communications link, whether or not a link with the desired service can be established or not.
An advantage of the invention is that as soon as the user has indicated his service need he is informed about whether the measured field strength meets the field strength requirement of the desired service. This way, unnecessary, unsuccessful connection attempts can be reduced.
Another advantage of the invention is that if the user insists to connect to a service that requires a great field strength because of weak channel coding, the transmission power of the mobile station can be increased in a controlled manner above the normal transmission power level for the duration of data transmission. The user may be charged extra for such a function.